This invention relates generally to an apparatus for determining the nitrogen oxide concentration in a gaseous mixture.
Such an apparatus has been disclosed in the German published application No. 24 07 133, see also U.S. Pat. No. 3,947,685. When it is desired to measure small concentrations of nitrogen oxide corresponding to an absorption coefficient A=0.1 in an absorption cell it is found that the normal equation controlling this process must be modified. The normal equation is: EQU I.sub.M /I.sub.V =K(1-A) (1)
wherein I.sub.M is the test signal also called unfiltered signal, I.sub.V the reference signal, A the absorption and K a constant. The modified equation is as follows: EQU .alpha.(p,T,t)I.sub.M /I.sub.V =K(1-A) (2)
wherein p is the pressure in the discharge lamp, T the ambient temperature and t the time. The time t indicates that the test signal may drift with time which may, for example, be caused by aging effects. The disturbance factor alpha is typically between 0.95 and 1.05. Assuming that A is 0.1, .alpha. produces within the limits given a measured error of .+-.50% in the nitrogen oxide concentration.
In case a discharge lamp is used which is filled with an oxygen-nitrogen mixture (for example, air) of reduced pressure, a pressure change in the lamp may be caused, for example, by a change in the ambient temperature or be due to a slow gas clean-up in the lamp. This pressure change causes a change of the emission characteristics of the gas discharge lamp. The lamp emits two fractions, one fraction being caused by NO molecules, the state of which corresponds to an occupation probability temperature of about 300.degree. K. (Kelvin) and another portion corresponding to an occupation probability temperature of about 1500.degree. K. Because the ratio of the two fractions depends on the gas pressure the observed measuring error is primarily due to this change.
Similar conditions prevail in the modulating device when a filter cell filled with NO gas is utilized. Finally the monochromator or an interference filter used instead is subject to aging and has a certain temperature dependence. This also causes errors of the final measuring result. These influences are combined in the disturbance factor alpha.
It is accordingly an object of the present invention to minimize or eliminate the influence of this disturbance factor in the region of low NO concentrations upon the final measuring result.